Variable (switched) reluctance motors are known. Variable reluctance motors conventionally have multiple poles or teeth on both the stator or rotor which are doubly salient. Phase windings are wound on the stator. No windings are wound on the rotor. As a result of the rotor not having windings, variable reluctance motors may be used in applications where reduced weight or high speed rotation is required as a consequence of the simplified rotor structure provided by a variable reluctance motor.
Torque is produced in a variable reluctance motor by switching current on and off in each of the phase windings in a predetermined sequence. The switching is synchronized with the angular position of the rotor so that a magnetic force of attraction results between the rotor and the stator poles. The current is switched off in each phase before the rotor poles nearest the stator poles of that phase rotate past the aligned position. Torque developed by the variable reluctance motor is independent of the direction of current flow. As a result, an inverter drive may be used to supply current for driving the phase windings wound on the stator poles.
The controller of a variable reluctance motor switches the stator phase currents on and off in synchronization with the rotor. By properly positioning the time intervals during which switches in an inverter drive the phase windings of the stator, forward or reverse torque may be obtained. Thus for applications such as driving an actuator on an airframe where the direction of rotation is essential for proper operation, machine tool applications, etc., it is necessary to determine the position of the rotor with respect to the stator poles to insure that the correct timing sequence of activating the switches in the inverter driving the phase windings is activated for clockwise or counterclockwise rotation. Conventionally, the position of the rotor is determined by a rotor position signal being applied to the controller from a shaft position sensor such as an encoder or a resolver. However, the provision of a shaft position sensor in a variable reluctance motor is expensive, creates reliability problems, and adds weight to the rotor and overall motor.
Prior art variable reluctance motors are known which do not utilize a shaft position sensor. U.S. Pat. No. 4,642,543 discloses that a variable reluctance motor may be started by the steps of aligning the motor rotor by energizing a predetermined stator phase, stepping the stator phases at about the starting rate of the motor for a predetermined starting delay, checking the average total current flowing in the motor and returning to the aligning step if the average exceeds a predetermined value, and ramping up the starting rate to a desired running frequency. Thus, the '543 patent solves the problem of determining the position of the rotor in order to produce a proper starting sequence by insuring that the rotor is aligned to a predetermined position prior to execution of the starting sequence. U.S. Pat. No. 4,772,839 discloses a variable reluctance motor which senses the position of the rotor during rotation by measuring current in two idle phases, processing the measurements to provide a pair of possible rotor angles for each such phase and combining the angles in a fashion which yields a unique estimate of the instantaneous rotor position. If two phases do not remain unenergized throughout the sampling period or if any phase of the rotor experiences a change of state during the sampling period, an extrapolated rotor position is calculated in place of the estimated instantaneous rotor position. The system of the '839 patent is much more complicated than that of the present invention.